1. Field of the Invention
This invention relates to a semiconductor device comprising transistors with gate all around (GAA) structures and a manufacturing method thereof.
2. Description of the Related Art
For example, in a memory cell of a spin-transfer type MRAM, one path is used both for writing and reading. Therefore, it is required to provide a large current in the write operation and a small leak current in the read operation. That is, in the MRAM, it is desired to set a high ON/OFF ratio for the cell transistor. As a cell transistor having a high ON/OFF ratio, a gate all around (GAA) transistor is proposed (for example, see Ming Li, et al., Int. Electron Devices Meet. 2007, 899).
The GAA transistor comprises a thin silicon (Si) nano-wire used as a channel. A gate electrode is formed to surround the Si nano-wire. Since the GAA transistor comprises a fine Si nano-wire as a channel, it is possible to pass a current through the channel without injecting an impurity. Therefore, since electrons that are moving are suppressed from colliding with impurities or the like and scattering within the GAA transistor, ballistic transport is realized.
However, since the parasitic resistance of the GAA transistor is high because of the fine structure, a sufficiently large current cannot be attained. For example, since the Si nano-wire used as a channel is fine, the contact area between the Si nano-wire and a contact plug becomes small. Accordingly, there occurs a problem that the contact resistance between the Si nano-wire and the contact plug becomes high, which reduces the drive current.